We reinvestigate the two-photon absorption of the (3s-n) transition in acetaldehyde and its implications for the observation of collision-induced alignment. The two-photon spectroscopy of an aligned ensemble of acetaldehyde produced in collisions with He provides clear evidence for the quantum interference of tensors of different rank. The observed polarization effect can be understood in terms of the non-vanishing tensor components Z(0)((0)) = 1.0, Z(0)((2)) = 0.4 +/-0.1 and \Z(2)((2))\ = 0.3 +/- 0.1. For a collision energy of 916 cm(-1) the alignment is largely consistent with the predictions of the kinematic apse model. (C) 2004 Elsevier B.V. All rights reserved.